The present invention relates to manufacturing optical articles out of thermoplastic synthetic material, such as ophthalmic lenses, instrument lenses or precision optics, as obtained by injection molding.
The molding of ophthalmic lenses out of thermoplastic synthetic material is usually performed by injection molding, with this method enabling raw plastics material to be transformed directly into finished lenses. In the manufacture of lenses by a method of this kind, it is conventional for the thermoplastic material to be initially heated so as to be molten at a temperature above the vitreous transition point. While in this form, the material is introduced under high pressure into a mold cavity of appropriate dimensions and shape that is formed in a mold. The material is then allowed to cool down so as to solidify, after which the resulting lens is extracted from the mold. Usually, the material used is a thermoplastic resin such as polymethyl methacrylate, polycarbonate, or a copolymer of polycarbonate, polynorbornene, polystyrene, cyclic polyolefins and their copolymers, etc.
To obtain ophthalmic lenses possessing optical qualities suitable for their purpose, certain precautions need to be taken during manufacture, in particular to avoid irregular deformations or the presence of residual internal tensions. Such deformations or tensions give rise to anisotropy or to other undesirable optical aberrations such as double refraction.
In this respect, particular care is taken in making the wall of the mold cavity in the mold. Usually, the mold cavity is made up of two shells each having a mold face of appropriate curvature corresponding to the curvatures that are to be given to a finished lens. The shells are generally made of stainless steel, and they present optical polish, i.e. analogous to that of a mirror.
It is often recommended to perform injection of the material into the mold cavity in two successive stages: a first stage of filling proper during which the mold cavity is filled progressively; and a second stage of overpacking or compression that occurs after the mold cavity has been filled completely. This second stage of overpacking or compression consists in subjecting the material that has been introduced into the mold cavity to high pressure for a given length of time and it is intended to eliminate shrink marks, to ensure that the material has the proper density, and to reduce any harmful internal tensions, at least to some extent. When this holding pressure is generated by the injection machine itself, the process is referred to as overpacking. When it is the result of the mold shells being moved towards each other, it is said to be compression.
These precautions relating to the tooling and the mode of operation need to be associated with precautions relating to heating of the plastics material and of the mold during molding. Usually, the mold is provided with channels for circulating a heat-conveying fluid to regulate the temperature of the wall of the mold cavity from one cycle to the next, and to accelerate the removal of heat from the molded lens.
In order to avoid surface cooling taking place before the end of injection, with a cold xe2x80x9cskinxe2x80x9d forming on the synthetic material that is injected into the mold cavity, proposals have been made in patent U.S. Pat. No. 5,376,317 (published Dec. 27, 1994) to proceed, prior to introducing material into the mold cavity, with heating of the wall of the mold cavity so as to bring it to a molding temperature which is higher than the vitreous transition temperature of said synthetic material. In particular, when the material is polycarbonate, which has a vitreous transition temperature of about 160xc2x0 C., the wall of the mold cavity is heated to a temperature of about 260xc2x0 C.
However, according to the teaching of that document, the heating of the mold cavity wall must be fully accomplished before any material begins to be injected into the mold cavity. The reason is that the fundamental purpose of the technique described in that document is to prevent the surface of the molten material solidifying while it is being injected, and in this way to prevent as much as possible any tension appearing during filling. According to that teaching, it is out of the question to attempt to relax such stresses, and on the contrary it is necessary to act upstream in order to prevent such stresses from appearing.
That technique gives rise to a major drawback: cycle time is considerately lengthened by running the heating and injection stages sequentially, given that their respective durations can be compressed very little. Furthermore, during filling and in spite of the wall of the mold cavity being heated, internal tensions are inevitably induced in the material by the high pressure and by the flow to which it is subjected. These tensions then become frozen in and sustained by the rapid cooling of the material which begins immediately after injection (i.e. without any pause after the overpacking or compacting stage).
An object of the invention is to further reduce the residual tensions within a lens molded in this way so as to improve its optical quality, but without lengthening cycle time, and possibly even reducing it.
To achieve this object in particular, the invention provides a method of injection molding an optical article such as an ophthalmic lens out of thermoplastic synthetic material by means of a mold containing a mold recess, said material being previously melted to a molding temperature higher than or equal to its vitreous transition temperature, the method comprising, for each lens molding cycle, the following steps:
raising the wall of the mold cavity to a heating temperature higher than or equal to the molding temperature of said material;
filling the molding cavity with said material;
at the end of filling, increasing the pressure of said material introduced in this way into the molding cavity up to a compacting pressure; and
bringing the wall of the molding cavity back to a cooling temperature to cool said molded material down to an unmolding temperature below its molding temperature, said cooling temperature being lower than said unmolding temperature;
in which method the filling of the molding cavity with said material begins before the heating temperature has been reached, and once both the compacting pressure of said material and the heating temperature of said molding cavity have been reached, they are both maintained for a given length of time.
The cycle time is thus reduced since the wall of the mold cavity continues to be heated in parallel with material being injected. Injection is then followed by a stage of maintaining the temperature during which the internal tensions that can arise during injection are relaxed. According to the invention, the essential point is for the molded synthetic material, and in particular its surface, to be maintained at a temperature higher than its vitreous transition temperature after injection has been performed, so as to allow the internal tensions in the material that result from it being injected to relax and so as to allow the material to be shaped under pressure inside the mold cavity. It is of little importance that the mold has still not reached its molding temperature before beginning to inject. Internal stresses will be relaxed in any event after injection has been completed. All of the physical and chemical properties of the synthetic material are thus conserved, or more precisely restored, without disturbance.
Filling can begin at the same time as the temperature of the mold wall cavity begins to be raised, or while it is being raised. Thus:
either the filling of the mold cavity with said material and the raising the temperature of the mold cavity wall both begin simultaneously; or else
the filling of the mold cavity with said material begins while the temperature of the mold wall cavity is already being raised; in which case, more precisely, filling is started less than 30 seconds (s) and/or more than 5 seconds after starting to raise the temperature of the mold wall cavity.
It can also be advantageous for the filling of the mold cavity with said material to terminate and for the pressure of said material to begin to be raised before the heating temperature has been reached. It is even possible to provide for the pressure at which said temperature is compacted to be reached before the wall of the mold cavity reaches the heating temperature.
Concerning the heating and cooling temperatures and the times at which they need to be acquired, experiments have shown that the following values when taken individually or in combination are advantageous, in particular when said material is polycarbonate:
the heating temperature lies in the range 30xc2x0 F. to 120xc2x0 F. above the vitreous transition temperature of the thermoplastic material used; for polycarbonate whose vitreous transition temperature is 300xc2x0 F., this corresponds to a heating temperature lying in the range 330xc2x0 F. and 420xc2x0 F.;
the cooling temperature lies in the range 20xc2x0 F. and 100xc2x0 F. below the molding temperature of said material; for polycarbonate whose vitreous transition temperature is 300xc2x0 F., this corresponds to a cooling temperature lying in the range 280xc2x0 F. and 200xc2x0 F.;
the time required to raise the temperature of the mold cavity wall from its cooling temperature to its heating temperature lies in the range 30 s and 150 s and is preferably about 60 s;
the time required to lower the temperature of the mold wall cavity from its heating temperature to its cooling temperature lies in the range 30 s and 150 s and is preferably about 60 s; and
the time during which the compacting pressure of said material and the heating temperature of the mold wall cavity is maintained after being reached is greater than 5 s and preferably lies in the range 10 s to 30 s.